Products of general formula (I) can be employed to prepare taxol under the conditions described in papers by J-N. Denis et al., J. Org. Chem., 51, 46-50 (1986); J. Amer. Chem. Soc., 110, 5917-5919 (1988) or the taxol derivatives which are described in European Patent EP 253,738.
It is known, in particular according to J-N. Denis et al., J. Org. Chem., 51, 46-50 (1986), to prepare the products of general formula (I) by titanium-catalyzed asymmetric epoxidation (T. Katsuki and K. B. Sharpless, J. Amer. Chem. Soc., 102, 5974-5976 (1980); J. G. Hill et al., J. Org. Chem., 48, 3607 (1983)) of cis-cinnamic alcohol, followed by oxidation and esterification of the epoxyalcohol obtained. However, the yields are not satisfactory, the enantiomer excesses are generally lower than 80% and the route is relatively long.
According to K. Harada, J. Org. Chem., 31, 1407-1410 (1966), it is known to separate the isomers of trans-.beta.-phenylglycidic acid by precipitation of a salt with optically active .alpha.-methylbenzylamine.
According to K. Harada and Y. Nakajima, Bull. Chem. Soc. Japan, 47 (11) 2911-2912 (1974), it is known to separate the isomers of cis-.beta.-phenylglycidic acid by precipitation of a salt with optically active ephedrine.
Application EP 0,342,903 describes the separation of an enantiomer of .beta.-(4-methoxyphenyl)glycidic acid from the corresponding diastereomer by means of optically active .alpha.-methylbenzylamine.
However, to make it possible for these processes to be carried out, the cis and trans isomers of .beta.-phenylglycidic acid must be separated beforehand.
It has now been found, and it is this that forms the subject of the present invention, that (2R,3R)-.beta.-phenylglycidic acid can be obtained from a mixture of the cis and trans isomers.